The present invention relates to photoconductive devices which absorb incident light. In particular, the present invention relates to solar cells in which light absorption is optimized by means of a rough surface whose geometry is correlated to the incident light.
Many of the materials used for solar cells, such as silicon, require a thick active layer because of low absorption of incident light. In particular, silicon has low solar absorptance in the infrared and far-infrared. It is, however, desirable to reduce the thickness of the active material for several reasons. First, a thin layer would reduce the amount and, therefore, the cost of the active material. Second, a thin layer would allow for a reduction in the diffusion length of the generated carriers. Shorter diffusion lengths allow for a reduction in the purity of the active material.
It is known that the optical absorption and, hence, the efficiency in the thin film solar cells is enhanced by texturing. Light scattered from a randomly textured surface (or surfaces) can be trapped in the active layer of the cell by internal reflection. The amount of light trapped depends upon the details of the texturing, see e.g., E. Yablonovitch and G. Cody, IEEE Transactions on Electron Devices ED 300, 1982.
Another way to increase the efficiency of thin-film solar cells is by using a grating substrate, see, e.g., U.S. Pat. No. 4,493,942 and U.S. Pat. No. 4,398,056. Such a device relies on the existence of guided modes in thin films, and the use of a grating to couple the incident light into these guided modes. This can result in extremely strong enhancement for absorption of light of a given frequency at a given angle of incidence. In other words, the diffraction grating is a strongly tuned device. For practical solar cells however, one is interested in a range of frequencies, determined by the solar spectrum and the absorption spectrum of the semiconductor, and in a range of incident angles, determined by the motion of the sun over the sky in the course of the day. Only a small fraction of this range can take advantage of the fine tuned grating.
The present invention is a new class of structures, denoted as "correlated roughness" structures, which combine both random and guide mode structures in order to optimize the coupling to externally incident sunlight over the relevant range of frequencies and angles.
For a given grating configuration, there can be only a discrete number of incident angles at which the coupling of the incident light with the guided modes is locally optimal. If the incident light were to deviate slightly from these optimal angles, the grating would have to be varied in order to maintain the optimal coupling. If one demands that a single structure maintain the most effective possible coupling as the incident angle were varied, such a structure could not be a grating, since a grating requires perfect periodicity. Rather, the structure which would maintain optimal coupling averaged over angles can be visualized as a weighted superposition of gratings of periodicity appropriate for each angle of interest. Such a structure is not completely random, since locally there is still a significant amount of ordering. Yet over a large enough distance the randomness is apparent.